1. Field of the Invention
This invention relates to noise cancellation techniques and, more particularly, to common-mode current cancellation techniques for isolated applications.
2. Description of the Related Art
A push-pull DC-DC converter may be used to generate isolated power supplies in a system including a non-isolated subsystem and an isolated subsystem. The transformer of the DC-DC converter may provide the power necessary for operation of the circuitry in the isolation subsystem. In addition, the transformer may provide isolation between the non-isolated subsystem and the isolated subsystem by blocking DC signals.
A typical problem with systems implementing isolation applications is the common-mode current that flows from the non-isolated primary to the isolated secondary through the parasitic capacitance inside the transformer. The common-mode current may interfere with the signals in the I/O channel of the isolated subsystem and may potentially cause interference in a customer's circuit or system that is connected to the isolated subsystem. One method to minimize the common-mode current is to place a grounded shield between the primary and secondary of the transformer. However, the grounded shield usually increases the transformer size, cost, and complexity.
Another technique to minimize the common-node current is to inject a compensation current of the opposite phase into the isolated ground by using a potentiometer and a resistor-capacitor (RC) network. This solution may apply a scaled version of the primary waveform to a capacitor connected to the isolated ground plane. The potentiometer may be used to weight one side of the primary more than the other. However, the potentiometer solution typically increases the parts count and cost of the system.